The US has claimed it has evidence of sarin gas use in Syria, after President Barack Obama said America "should take military action" following an alleged chemical attack in the country.

Secretary of State John Kerry used interviews with several US news channels to suggest the case for military strikes is growing.

He said samples of hair and blood provided by first responders in Damascus had tested positive for sarin, a nerve agent estimated to be 500 times more toxic than cyanide.

UN weapons inspectors have been asked by Secretary-General Ban Ki-moon to speed up the analysis of samples they collected in the city, a UN spokesman confirmed.

Image Caption:UN weapons inspectors gathered evidence form the site of the alleged attack

Meanwhile, Syrian president Bashar al Assad told state TV his country is capable of confronting any attack.

"The American threats of launching an attack against Syria will not discourage Syria away from its principles ... or its fight against terrorism supported by some regional and Western countries, first and foremost the United States of America," he said.

"Syria ... is capable of facing up to any external aggression just as it faces up to internal aggression every day, in the form of terrorist groups and those that support them."

Syria generally refers to rebels fighting to topple the leader as "terrorists".

According to US estimates, up to 1,429 people, including at least 426 children, were killed in the alleged chemical attack in Damascus last month.

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Former Iranian president Akbar Hashemi Rafsanjani told the semi-official Iranian Labour News Agency that Syrians were targeted "by their own government" and said they "must also wait for an attack by foreigners."

"The people of Syria have seen much damage in these two years," he added.

Arab states urged the UN and international community to take action against the culprits of the alleged attack, calling on world leaders to take "deterrent and necessary measures".

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It came after Saudi Arabia's foreign minister Saud al-Faisal said it was time for "the international community with all its power to stop this aggression against the Syrian people", adding the country would back a US strike on Syria if the Syrian people did.

Mr Kerry said he was confident Congress would give Mr Obama its backing for a military strike against Syria when members return from their summer break on September 9.

But the former senator said the president had the authority to act on his own if Congress does not give its approval.

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Syria's Deputy Foreign Minister Faisal Muqdad told reporters Mr Obama had grown "hesitant ... and confused" after he postponed the action until after the vote.

The Syrian main opposition bloc said it was "disappointed" with Mr Obama's decision to wait for Congress but said it believed lawmakers would approve a strike.

Earlier William Hague ruled out a second vote on British military involvement in Syria.

He told Sky's Murnaghan programme that "Parliament has spoken" and said it would be unrealistic to try once more to persuade MPs to support military force.

He also branded the Labour party "opportunistic" and said its MPs had some "hard thinking to do" after the stance they took in Thursday's vote.

Meanwhile, Pope Francis has condemned the use of chemical weapons and announced he would lead a worldwide day of fasting and prayer for peace in Syria on September 7.

He abandoned the traditional religious theme of the weekly papal appearance to crowds in St Peter's Square on Sunday and instead spoke entirely about Syria.

He said: "My heart is deeply wounded by what is happening in Syria and anguished by the dramatic developments."

Pope Francis reiterated appeals for all sides in the civil war to put down their arms and "listen to the voice of their conscience and with courage take up the way of negotiations".